Exhaust system

ABSTRACT

An exhaust system or an engine ( 12 ) includes a lean NOx catalytic device ( 18 ), and a heat exchanger ( 70 ) positioned upstream of the catalytic device ( 18 ). Control means ( 44, 46 ) controls a valve ( 36 ) to regulate exhaust gas flow through the heat exchanger ( 70 ) or along a bypass path ( 26 ). The heat exchanger ( 70 ) can cool the exhaust gases to ensure that the maximum operating temperature of the catalytic device ( 1 ) is not exceeded. During use, the heat exchanger ( 70 ) can be bypassed to allow high temperature purge cycles.

[0001] This application is a continuation of U.S. utility patentapplication Ser. No. 09/700,484, filed on Feb. 21, 2001.

BACKGROUND AND SUMMARY

[0002] The present invention relates to an exhaust system for aninternal combustion engine, in particular to an exhaust system employinga catalytic device for purifying the exhaust gases. The invention isespecially suitable for a system for a lean burn engine (employing alean NOx catalytic device), but it is not limited exclusively to this.

[0003] In general terms, the need to operate a catalytic device above aminimum operating temperature is well known in the art. For example,EP-A-0460507, GB-A-2278068 and WO 96/27734 describe arrangements forrouting the exhaust along an appropriate exhaust path if the gas is notat an optimum high temperature, or if the catalytic devices have not yetreached their optimum temperatures.

[0004] The increasing cost of fuel and the concern over CO₂ emissionshas lead a drive for engines with improved fuel economy. Lean burnengines have been developed using gasoline direct injection and portinjection techniques.

[0005] Under these lean operating conditions the standard 3-way catalystis very efficient for CO and hydrocarbon (HC) oxidation, but thereduction of oxides of nitrogen NOx (NO and NO₂) to di-nitrogen (N₂) isconsiderably more difficult. Catalytic converters and traps are beingdeveloped which can operate under lean conditions. The “lean” problem isthat, there is generally an insufficient quantity of hydrocarbons in theexhaust gas to enable efficient conversion of all of the NOx todi-nitrogen at the catalytic device. One type of lean burn engine uses alean cycle and an intermittent stoichiometric or rich cycle. A catalytictrap can be used which absorbs the excess NOx gases during the leancycle, and then converts the NOx to NO₂ in the presence of morehydrocarbons during the rich cycle. The rich cycle is sometimes referredto as the “purge” cycle.

[0006] Although lean NOx catalytic converters and traps offerpotentially enormous emissions benefits, it has been extremely difficultto attain the full potential of the catalytic devices, especially underconditions in which the engine is working hard (for example, for highspeed vehicle cruising). The reason is that, under such conditions, thetemperature of the exhaust gas entering the catalytic trap often exceedsthe optimum operating range for the catalytic device. For example, FIG.17 illustrates the typical temperature characteristics for a lean NOxtrap. The catalytic material has a coating for absorbing the excess NOx,but this is only effective up to about 450° C. On the other hand, thereduction of the oxides in the presence of hydrocarbons is onlyeffective at temperatures above about 200° C. This creates a usefultemperature window from approximately 22-450° C. in which the lean NOxconversion can occur. At temperatures outside this window (for example,caused by high engine speed), the catalytic trap will not operateefficiently. Lean NOx catalytic converters also operate in a similartemperature range.

[0007] Broadly speaking, one aspect of the present invention is toprovide a cooling heat exchanger unit upstream of a catalytic device,and a control device for providing selective cooling of the exhaust gasupstream of the catalytic device, using the heat exchanger.

[0008] With the invention, the heat exchanger unit can providesufficient cooling to cool the hot exhaust gases to a desired catalyticoperating temperature, or to within a desired operating temperaturewindow, for efficient catalytic operation.

[0009] Moreover, cooling of the exhaust gases provides other performanceadvantages, specifically by reducing the volume of the gas, and thus thevolume flow rate through the exhaust system. This can help reduce thebackpressure within the exhaust system, and can also help reduce flownoise through the system, especially at high engine speeds and loads.These are significant problems associated with lean NOx catalyticdevices, which tend to require relatively large substrates for efficientlean NOx operation. The use of large substrates can cause undesirablebackpressure build up. The reduction in backpressure will help toimprove fuel economy and reduce CO₂ emissions.

[0010] The heat exchanger unit may be a gas cooled unit (for example,air cooled), or it may be liquid cooled. The latter is preferred for thefollowing reasons:

[0011] (a) A liquid-cooled heat exchanger can avoid the occurrence oftransient temperature drops which air-cooled exchangers can cause.Initially, an air-cooled heat exchanger will be much colder than the hotexhaust gases and, when the hot gases first pass through the exchanger,the large temperature difference can causes a very efficient heatsinkeffect to occur. Such large transiens can cause the temperature to fallbelow an optimum operating range of the catalytic device until the heatexchanger heats up to near the exhaust gas temperature.

[0012] (b) A liquid-cooled heat exchanger remains at the temperature ofthe coolant, and never heats up to the exhaust gas temperature. Heattransfer is achieved through the large heat capacity of the liquid, anddoes not depend (at least to much extent) on the precise temperature ofthe coolant itself. In contrast, an air-cooled exchanger necessarilyheats up to near the exhaust gas temperature, and dissipates heat bybeing much hotter than the surroundings. This can cause design problemsfor placement on a vehicle away from hazardous (temperature sensitive)areas, and also requires the presence of a cooling air flow, in use

[0013] (c) A liquid-cooled heat exchanger can enable the use of anopen-loop control system for controlling the cooling operation withouthaving to measure directly the temperature of the exhaust gas in theexhaust system. Most vehicles are not equipped with an exhausttemperature sensor, and the addition of such a sensor able to withstandharsh exhaust conditions represents additional expense. With aliquid-cooled system, the exhaust gas temperature can be predicted usingthe outputs from conventional vehicle sensors for sensing, for example,the engine inlet air temperature, the engine coolant temperature, theengine speed, the air mass flow entering the engine, and the fuel:airmixture (measured using a lambda sensor).

[0014] (d) A liquid heat exchanger can generally be made more compactthan an air-cooled heat exchanger.

[0015] If a liquid heat exchanger is used, then preferably, this iscoupled to an existing coolant circuit of a vehicle, such as, forexample, the engine coolant circuit.

[0016] If a gas-cooled heat exchanger is used, then the arrangementshould comprise a gas inlet tube, a heat exchanger unit coupled to theinlet tube, and an outlet tube exiting the heat exchanger unit, the heatexchanger unit having a greater heat dissipation effect than the inletand outlet tubes.

[0017] In either type of system, the exhaust system preferably comprisesa first flow path through the heat exchanger for cooling the gas in thefirst path, and a second flow path bypassing the heat exchanger. Thesecond flow path may flow through the housing of the heat exchangeralong a substantially non-heat exchange (or at least a low-heatexchange) path.

[0018] In another broad aspect, the invention provides a method, andalso a control apparatus, for controlling operation of a cooling devicefor cooling exhaust gas upstream of a catalytic exhaust purificationdevice.

[0019] In one preferred aspect, the method includes predicting theexhaust gas temperature from a plurality of characteristics which areeach not directly indicative of the exhaust temperature, and controllingcooling operation in response to the predicted exhaust gas temperature.

[0020] In another preferred aspect, the method includes controlling thecooling during a first engine cycle to achieve an exhaust temperaturewithin a first operating range for the catalytic device, and during asecond engine cycle to achieve an exhaust temperature within a secondoperating range for the catalytic device.

[0021] The second operating range (achieved after the first operatingrange) may include a higher maximum temperature than the first operatingrange. For example, the second operating range may correspond to ashoichiometric cycle, or to a sulphur purge cycle. The first cycle maycorrespond to a lean cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments of the invention are now described by way of exampleonly, with reference to accompanying drawings, in which:

[0023]FIG. 1 is a schematic view illustrating a first embodiment of anexhaust system for a lean bum engine;

[0024]FIG. 2 is a schematic view illustrating the heat exchanger in moredetail;

[0025]FIG. 3 is a schematic view illustrating a comparative prior artexhaust system;

[0026]FIG. 4 is a graph illustrating gas temperatures during steadystate cruising;

[0027]FIG. 5 is a graph illustrating the improvement in catalyticconversion efficiency;

[0028]FIG. 6 is a graph illustrating the behaviour of the system of FIG.1 during a drive cycle;

[0029]FIG. 7 is a graph comparing engine torque and power in the heatexchanger valve-open and valve-closed positions;

[0030]FIG. 8 is a schematic view illustrating a second embodiment ofexhaust system;

[0031]FIG. 9 is a schematic section through the heat exchanger used inFIG. 8;

[0032]FIG. 10 is a plan view in isolation of a baffle for the heatexchanger of FIG. 9;

[0033]FIG. 11 is a plan view in isolation of an end plate of the heatexchanger of FIG. 9;

[0034]FIG. 12 is a graph illustrating the performance of the secondembodiment;

[0035]FIG. 13 is a more detailed view of a portion of FIG. 12illustrating the effect of coolant temperature;

[0036]FIG. 14 is a schematic section through an alternative design ofheat exchanger usable in the embodiment of FIG. 7;

[0037]FIG. 15 is a flow diagram illustrating the steps used to controloperation of the exhaust system;

[0038]FIG. 16 is a schematic diagram illustrating a control algorithm;and

[0039]FIG. 17 illustrates conversion efficiency of a conventional leanNOx catalytic trap.

DETAILED DESCRIPTION OF THE DRAWINGS

[0040] Referring to FIG. 1, a test exhaust system 10 is illustrated fora lean bum engine, identified schematically at 12. The exhaust systemcomprises an exhaust manifold 14 coupled to the exhaust ports of theengine 12, a conventional light-off catalytic converter 16 arrangedclose to the engine 12 to provide catalytic purification when the engineis first run, and a lean NOx catalytic device 18 arranged downstream ofthe light-off converter 16. The lean NOx device 18 may either be acatalytic trap, or a lean catalytic converter, to suit the engine 12.

[0041] Arranged between the light-off converter 16 and the lean NOxdevice 18 is a cooling arrangement 20 which consists of a heat exchangerunit 22 arranged in a first gas flow path 24, and a second gas flow path26 bypassing the heat exchanger unit 22.

[0042] Referring to FIG. 2, in this embodiment the heat exchanger unit22 is air cooled, and comprises a linear radiator arrangement of ninesteel exchanger tubes 28 extending between an inlet manifold tube 30 andan outlet manifold tube 32. The exchanger tubes 28 are cooled by movingair, represented by the fan 34 (FIG. 1).

[0043] In the illustrated test arrangement, the exchanger tubes 28 areapproximately 600 nun long, with an inside diameter of about 22 mm. Thefan 34 provides an ambient air speed of about 2.5 m/s over the heatexchanger unit 22.

[0044] Flow through the first and second paths 24 and 26 is controlledby a valve 36 situated in the first flow path 24 downstream of the heatexchanger unit 22. The flow resistance of the second path 26 relative tothe first path 24 is such that, when the valve 36 is open, a substantialportion of the gas flows through the first path 24 through the heatexchanger 22. When the valve 36 is closed, the gas has to flow throughthe second path 26, and thereby bypasses the heat exchanger 22. The flowrates through the first and second paths are selected such that neitherpath presents too high an impedance, which would otherwise causeundesirable back pressure in the exhaust path.

[0045] In the test arrangement illustrated in FIG. 2, the impedance ofthe second path 26 is made adjustable by means of a replaceableconstriction assembly 38. The assembly 38 consists of two flanges 40between which is received an exchangeable disc 42 having an orifice of apredetermined size.

[0046] The valve 36 is a vacuum controlled butterfly valve, which iscontrolled by means of an electrical solenoid 44. The solenoid iscontrolled by a control unit 46, described further below.

[0047] The above valve control arrangement is preferred, as it avoidsthe need to place a valve in the direct flow of very hot exhaust gases.Instead, the valve 36 is placed downstream of the heat exchanger unit,and so is exposed to less hot exhaust gas. This can increase valve life,and enable a less expensive valve to be used. However, it will beappreciated that in other embodiments, a flow switching valve may beused in the second flow path 26 if desired, or at one of the junctionsbetween the first and second flow paths 24 and 26 if desired. The valvemay be a butterfly type or other type of valve, as appropriate.

[0048] In this embodiment, a temperature sensor 48 measures the exhaustgas temperature upstream of the lean NOx catalytic device 18. Forexample, the temperature sensor 48 may be located at the inlet to thedevice 18, or upstream of the heat exchanger unit 22. The control unit46 may, for example, be a straightforward threshold sensing unit (withhysteresis if desired) which controls the valve 36 to open when theexhaust gas exceeds a threshold temperature, so that the temperature ismaintained in a desired temperature window. Alternatively, the controlunit 46 may include a predictive control 6 algorithm representing athermal model of the exhaust system to predict the exhaust gastemperature depending on the load conditions of the engine.

[0049] To test the effect of the heat exchanger, the same exhaust systemwas also used in a conventional test arrangement, as illustrated in FIG.3. Referring to FIG. 3, features described above are denoted by the samereference numerals, where appropriate. In this conventional testarrangement, the heat exchanger of FIG. 1 is replaced by a steel tubeapproximately 750 mm long. This is equivalent to the path length theexhaust gas travels when the valve 36 of FIG. 1 is closed. This pipelength is also representative of the typical distance between a closecoupled (light-off) catalytic converter in a vehicle engine bay, and anNOx trap in an underfloor position on a vehicle.

[0050]FIGS. 4, 5 and 6 illustrate the performance comparisons betweenthe arrangements of FIGS. 1 and 3. The engine used was a 1.8 literfour-cylinder homogeneous lean bum engine coupled to a 100 KW DCdynamometer, to simulate appropriate loading on the engine.

[0051]FIG. 4 illustrates the exhaust gas temperature at the inlet of thelean NOx catalytic device 18 at an engine speed and load correspondingto vehicle cruising at a speed of 120 Km/h (about 75 mph). Bar 50represents the temperature for the conventional system of FIG. 3,reaching about 600° C., which is well outside the operating window of200-450′C for the catalytic device 18. With the heat exchanger unit 22in place, and the control valve 36 open, the temperature is reduced toabout 424′C as illustrated by bar 52, which is inside the optimumtemperature range.

[0052]FIG. 5 illustrates the NOx conversion efficiency of the lean NOxdevice 18 for the above conditions. For an exhaust gas temperature ofabout 600′C, bar 54 shows that the conversion efficiency is less than10%, resulting in high NOx pollution. However, for the lower exhaust gastemperature achieved with the heat exchanger unit 22, bar 56 shows thatthe conversion efficiency approaches 50%.

[0053]FIG. 6 illustrates the exhaust gas temperature (at the inlet tothe lean NOx catalytic device 18) over the first 1200 seconds of thestandard reference European drive cycle.

[0054] Line 58 illustrates the temperature for the conventional exhaustarrangement of FIG. 3. In the urban drive cycle (portion 60), thetemperature reaches the minimum operating 7 temperature of 2001C for thelean NOx catalytic device 18 after about 150 seconds. The temperatureremains below the maximum threshold of 450′C throughout the urbanportion of the drive cycle (portion 60). However, during the extra urbanportion (portion 62), the temperature quickly exceeds the maximumthreshold of 450′C.

[0055] Line 64 illustrates the catalytic device inlet temperature forthe exhaust arrangement of FIG. 1. In the urban drive cycle portion 60,the temperature reaches the minimum lean NOx catalytic operatingtemperature after about 250 seconds, the gas exhaust temperature beingabout 50′C below that with the exchanger unit 22 removed, even thoughduring this portion of the cycle the valve 36 is closed. Thistemperature reduction is believed to be a result of direct heatconduction through the metal tubes of the exhaust system, resulting insome heat loss through the heat exchanger unit 22. In the extra urbanportion 62 of the cycle, the temperature begins to rise, resulting inthe valve 36 opening to allow gas through the heat exchanger unit 22.The temperature falls abruptly, and remains below the 450″C threshold.

[0056] As described previously, the gas flow rates through the first andsecond flow paths 24 and 26 (FIGS. 1 and 2) are designed such that theflow distribution can be controlled by a single valve 36 downstream ofthe heat exchanger unit 22. FIG. 7. illustrates a comparison of theengine power and torque curves for the open and closed conditions of thevalve 36. Any large variation in engine performance would be veryundesirable, as this would affect the drivability of the vehicle,depending on whether the valve were to be open or closed. However, ascan be seen, there is very little change in the engine performance whenthe valve is switched.

[0057] It will be appreciated that the cooling arrangement illustratedabove can provide significantly better NOx conversion performancecompared to a conventional exhaust arrangement. The use selectivecooling (provided above by two flow paths) can ensure that cooling isonly used when needed, i.e. when the exhaust gas temperature becomeselevated. During initial running of the engine (and during NOx purge andsulphur purge cycles), the cooling can be bypassed, to ensure that thelean NOx catalytic device 18 reaches the desired operating temperature,or purge temperature, quickly.

[0058] 8 A further and important benefit in cooling the exhaust gases isthat it inherently reduces the volume of the gas, and the thus thevolume flow rate of the gas through the exhaust system. Such a reductioncan reduce back-pressure and also the flow noise in the exhaust system.Back-pressure in a lean NOx system is a very important consideration,because the catalytic substrates used for the lean NOx catalytic devicesgenerally have to be relatively large to provide good performance inlean conditions. Such large substrates can result in a back-pressureincrease, and so any means of reducing the back-pressure is highlydesirable.

[0059] One of the features of the air-cooled heat exchanger systemdescribed above is that there tends to be a large transient temperaturedrop when the control valve 36 is switched to the open condition. Such atransient drop is visible in FIG. 6 at point 66. This is a result of theheat exchanger unit 22 being initially very cool (since it is cooled bythe fan 34), and acting as a very efficient heatsink when the exhaustgas is first passed through the heat exchanger 22. As more exhaust gaspasses through the heat exchanger 22, the heat exchange tubes 28 heatup, and provide a lesser rate (by dissipating the heat in the air streamprovided by the fan 34). Such a transient may be undesirable, as it cancause the exhaust gas temperature to fall below the minimum activationtemperature for the lean NOx catalytic device 18 (about 200′C), forexample as illustrated by the point 66 in FIG. 6.

[0060] FIGS. 8-11 illustrate a second embodiment, which can provide allof the advantages of the first embodiment, and also addresses thetransient problem. Where appropriate, the same reference numerals havebeen used to denote features equivalent to those described previously.

[0061] The principle difference in FIG. 8 is the use of a liquid-cooledheat exchanger unit 70 in place of the air-cooled heat exchanger unit 22of FIG. 1. The liquid-cooled heat exchanger 70 consists generally of ahollow housing 72 which, in this embodiment, is cylindrical and containsan arrangement of gas carrying tubes 74 arranged as a uniform “bundle”,with spacing between adjacent tubes to allow thermal contact with thesurrounding coolant liquid. The tubes 74 extend between two end plates76 which are 3o apertured to define an openings 77 into which each tube74 opens at its end. The ends of 9 the tubes 74 are welded to the endplates in a liquid-tight manner. Outside the end plates 76, the housingdefines an inlet chamber 78 to allow the incoming exhaust gas to bedistributed to flow into the tubes 74, and an outlet chamber 80 for there-collimation of the gas flowing out of the tubes 74.

[0062] The housing 72 defines a liquid-tight chamber surrounding thetubes 74. Liquid coolant is received through a coolant inlet port 82 andis circulated in the housing before exiting through a coolant outletport 84. In order to ensure optimum flow of the coolant in contact withthe tubes 74, the housing includes a plurality of internal baffles 86.Each baffle is similar to the end plates 76 in that it consists of awall with openings 88 through which the tubes 74 pass. However, eachbaffle includes a “cut-away” portion to define a passage between theedge of the baffle and the housing to permit the flow of liquid aroundthe baffle. As best seen in FIG. 9, the baffles 86 are arTangedalternately to define a tortuous sinusoidal flow path for the coolantliquid between the inlet and outlet ports 82 and 84.

[0063] In the present embodiment, the heat exchanger 70 is made ofsteel, and is relatively compact, including 19 tubes 74 each of length440 mm and diameter 14 mm.

[0064] The housing has a diameter of about 88 mm, and the baffles eachhave a “height” of about 60 mm. The baffles are arranged with a uniformspacing of about 110 mm, and are secured in position by being spotwelded to, for example, three of the tubes 74.

[0065] Liquid coolant circulated through the heat exchanger 70 by aliquid coolant circuit 90 which includes a heat dissipating radiator 92and a coolant pump 94. The coolant circuit may be a dedicated circuit inthe vehicle, but in this preferred embodiment, the coolant circuit ispart of an existing coolant circuit on the vehicle, for example, theusual engine coolant circuit and using the engine radiator (92) and theengine coolant pump (94). This can avoid the additional space and costof using an independent cooling circuit.

[0066]FIG. 12 illustrates the performance of the exhaust system with theliquid-cooled heat exchanger, and using a similar engine and testarrangement as that described previously. In FIG. 12:

[0067] the line 96 represents the temperature of the exhaust gases atthe inlet to the heat exchanger (equivalent to the exhaust gastemperature reaching the lean NOx catalytic device 18 if the heatexchanger were to be omitted); the line 98 represents the temperature ofthe exhaust gas leaving the heat exchanger (equivalent to thetemperature of the exhaust gas entering the lean NOx device 18 when thecontrol valve 36 is open); the line 100 represents the temperature ofthe liquid coolant being circulated through the heat exchanger; and theline 102 represents the mass flow of the exhaust gases.

[0068] The graph illustrates the measured characteristics over a cycleincluding three different engine settings, the first portion 104 beingat an engine speed of 1000 rpm at % throttle, the second portion 106being at an engine speed of 2000 rpm at 50% throttle, and the thirdportion 108 being at an engine speed of 4000 rpm at 100% throttle.

[0069] As can be seen from the graph, the relatively compact heatexchanger provides adequate cooling to maintain the exhaust gastemperature below about 450′C even at elevated inlet temperatures, andhigh mass flow.

[0070] Moreover, the liquid heat exchanger does not produce anytransients when the flow of the exhaust gas is switched from the bypasspath to the heat exchanger path. This is because, unlike air-cooling,the wall temperature does not vary much. Rather, it is the high specificheat capacity of the coolant liquid which enables heat to be absorbed bythe coolant, with little resultant temperature dependency. For example,referring to FIGS. 12 and 13, in the portion 106 of the test cycledescribed above, the water temperature in the heat exchanger fluctuatesbetween about 80′C and 90′C. However, there is virtually no resultantchange in the gas outlet temperature from the heat exchanger (line 98).

[0071] A further advantage with a liquid coolant heat exchanger is that,in contrast to an air-cooled exchanger, the exchanger does not heat upto the high exhaust gas temperatures. The exchanger remains at thetemperature of the coolant. This can avoid the need to provide hightemperature dissipation devices in the exhaust system, which might provehazardous or position critical for underfloor exhaust systems, or forengine-bay exhaust components. The lack of any requirement for a coolingair flow over the 11 exchanger also permits the designer greaterflexibility in positioning the exchanger on a vehicle.

[0072]FIG. 14 illustrates an alternative design of liquid coolant heatexchanger 110, which incorporates the bypass, non-heat exchange path,within the housing 112 of the heat exchanger 110. This avoids the needto employ separate conduits for the exhaust bypass path. Referring toFIG. 14, the housing 112 through which the coolant flows has a generallyannular shape, and the heat exchange tubes 74 are arranged in an annularconfiguration within the housing 112. The central hollow of the housingprovides the bypass path 26 with little, or no, thermal contact with thecoolant medium. The heat exchange and non-heat exchange paths join ateither end of the housing 112 at an inlet chamber 114 and an outletchamber 116. The valve 36 is arranged within the bypass path and, inthis embodiment, can be an integral part of the heat exchanger unit.

[0073] If desired, it is possible to concatenate the above heatexchanger 110 with a catalytic device within a common housing, toprovide a single unit which contains a catalytic device and atemperature regulating mechanism.

[0074]FIG. 15 illustrates a typical control process loop 120 forcontrolling the valve 36 during the lean, rich and sulphur purge cyclesof the engine. Step 122 determines whether the engine is running and, ifnot, the process branches to a termination step 124.

[0075] If the engine is running, step 126 determines whether a sulphurpurge is necessary to clear the exhaust system of a build up of sulphuroxides. In. some countries, fuel contains a fairly high sulphur content,and the sulphur oxides tend to collect in the catalytic devices (and actin competition to the conversion of nitrogen oxides). The build up ofsulphur oxides is countered by a high temperature purge. If a sulphurpurge is necessary, then step 126 branches to step 128 at which a targettemperature window defined by Tmax, Tmin is set to correspond to thedesired high temperature for a sulphur purge, generally between about600″C and 750′C. Step 130 controls the valve 36 to try to achieve atemperature within the window. Generally, the desired temperature is sohigh that the valve 36 will remain closed during this period to allowthe exhaust temperature to reach maximum levels.

[0076] 12 Step 132 determines whether the sulphur purge has beencompleted. If not, the process loops back to repeat steps 128 and 132until completion of the sulphur purge.

[0077] Once the sulphur purge has been completed, or if no sulphur purgewas determined to be necessary at step 126, the process proceeds to step134 which determines whether the engine is currently running lean. Ifthe engine is running lean, then the process proceeds through step 136at which a target temperature window defMed by Tmax, Tmin is set tocorrespond to the temperature range for lean NOx catalytic operation,generally between about 200′C and 450′C. If the engine is not runninglean, then the target temperature window is set at step 138 tocorrespond to stoichiometric NOx catalytic operation, generally between350′C and 750′C.

[0078] The process then proceeds to step 137 which controls the valve 36to try to achieve a temperature within the target window. Thereafter,the process loops back to step 122 described above.

[0079] The valve 36 may be controlled either to be fully open of fullyclosed.

[0080] Alternatively, the valve 36 may be controlled to be open by acontrollable amount, through the use of proportion control, for examplePID (proportional integral differential) control.

[0081] The valve 36 may be controlled simply through the use of atemperature sensor which measures directly the temperature of the gas inthe exhaust system (closed loop feedback). However, the use of a liquidcooled heat exchanger system also permits an open loop control to beused which predicts the temperature of the exhaust gas without having tomeasure the exhaust temperature directly. This can provide cost savingsin not having to use a relatively expensive exhaust gas temperaturesensor.

[0082] An open loop system is illustrated, for example, in FIG. 16. Thesystem uses the outputs of sensors which are provided as standardsensors on most modem vehicles.

[0083] These are: an air temperature sensor 140 which provides a signalindicative of the inlet air temperature to the engine; a coolanttemperature sensor 142 which provides a signal indicative of the enginecoolant temperature; an engine speed sensor 144 which provides anindication of the rpm engine speed (as measured or as deduced from theengine control system); and air mass flow sensor 146 which provides asignals indicative of the air mass 13 flow into the engine; and a lambdasensor 148 which provides a signal indicative of the air: ftiel ratio asmeasured from the exhaust gases.

[0084] An engine map/model 150 is used to calculate the exhaust gastemperature and the exhaust gas mass flow from the engine, and anexhaust system thermal model 152 is then used to calculate the amount ofcooling required to bring the exhaust gas temperature to within thetarget temperature window, based on the liquid coolant temperature (forexample, the same as the engine coolant temperature if a common system).

[0085] The engine map/model 150, and the thermal model 152 of theexhaust system (including the heat exchanger), can be implementedrelatively easily using a computer based control system, for example, amicro controller.

[0086] It will be appreciated that the invention, particularly asdescribed in the preferred embodiments, can provide a system forcontrolling the temperature of exhaust gases to within the desiredoperating temperature window for a catalytic device.

[0087] It will be appreciated that the above description is merelyillustrative of preferred embodiments of the invention, and that manymodifications may be made within the scope of the invention. Featuresbelieved to be of particular importance are defined in the appendedclaims. However, the Applicant claims protection for any novel featureor aspect described herein and/or illustrated in the drawings, whetheror not emphasis has been placed thereon.

The claimed invention is:
 1. A method of operating an exhaust system,the system comprising a catalytic device and means for cooling exhaustgases in the system, the method comprising the steps of: controlling themeans for cooling of the exhaust so as to achieve an exhaust temperaturewithin a first operating range for the catalytic device appropriate to afirst engine cycle; and controlling the means for cooling of the exhaustso as to achieve an exhaust temperature within a second operating rangefor the catalytic device appropriate to a second engine cycle, thesecond operating range having a higher maximum temperature than thefirst operating range.
 2. A method of operating an exhaust systemaccording to claim 1 in which the first engine cycle is a lean cycle. 3.A method of operating an exhaust system according to claim 1 or 2 inwhich the second engine cycle is a stoichiometric cycle.
 4. A method ofoperating an exhaust system according to claim 1 or 2 in which thesecond engine cycle is a sulphur purge cycle.
 5. A method of operatingan exhaust system according to any preceding claim, the methodcomprising the further step of controlling the means for cooling of theexhaust so as to revert to the first operating range.
 6. A method ofoperating an exhaust system according to claim 1 in which the firstoperating range of exhaust temperature is from about 200 degrees C. to450 degrees C.
 7. A method of operating an exhaust system according toclaim 1 in which the second operating range of exhaust temperature isfrom approximately 350 degrees C. to 750 degrees C.
 8. A method ofoperating an exhaust system according to claim 1 in which the secondoperating range of exhaust temperature is from about 600 degrees C. to750 degrees C.
 9. An exhaust system for an internal combustion enginecomprising a catalytic device for purifying exhaust gases, a heatexchanger in a gas flow path upstream of the catalytic device, controlmeans for controlling the cooling of the exhaust gases by the heatexchanger, the control means controlling the cooling of the gases so asto maintain the exhaust gases at a temperature within a first operatingrange and, when required, controlling the cooling of the exhaust gasesby the heat exchanger so that the exhaust temperature lies within asecond operating range having a higher maximum temperature than thefirst operating range.